1. Field of the Invention
The present invention relates to an apparatus for reproducing data recorded in optical disk.
2. Description of the Prior Art
The optical disk is nowadays an essential external storage medium to computers. It is known that the recording system for optical disks has two kinds: one is a pit-position recording system and the other is an edge-position recording system. The pit-position recording system has record data corresponding to the center of a written record pit, and the edge-position recording system has record data corresponding to each edge of a record pit, which is known to be more advantageous than the pit-position recording system in that it can store a greater quantity of information per unit region and have greater recording density.
The optical disk, whether it is based on either recording system, requires a data reproduction apparatus. The data reproduction apparatus must recognize which recording system the optical disk is based on. To this end, JAP3-144919 and JAP4-79031 propose a compatible data reproduction apparatus which can be used when the optical disk is based on either recording system.
JAP3-144919 obtains a distinguishing information from the optical disk or from the outside whereby either recording system is identified. As a result, the data is reproduced in an analog system suited to the identified recording system. JAP4-79031 is also directed to the identifying of either of recording systems, and provides a method for identifying "which system" by a mark physically made on the optical disk such as the casing. When the optical disk is loaded on an apparatus, the mark functions as a distinguisher, thereby recognizing that the optical disk is based on a pit-position record system or an edge-position record system. The data is reproduced in an analog system suited to the identified recording system. A common feature with these conventional reproduction apparatus is that they presuppose that the data is reproduced in an analog system.
The analog reproducing system inherent to them makes it impossible for a PRML (partial response maximum likelihood) to be applied to, which will be described in greater detail below.
As a new reproducing system of edge-position recorded data, a digital data reproducing technology called PRML (partial response maximum likelihood) is being studied. This PRML reproducing technology is a technology for demodulating the information modulated and recorded in accordance with partial response characteristic by a maximum likelihood method (Viterbi decoding), and it is superior in the detection capability to the conventional analog reproducing system. First, record data is limited in run length, and the signal modulated in accordance with the partial response characteristic is recorded in an optical disk, and a reproduction signal obtained from the optical disk is sampled by an analog/digital (A/D) converter, and the transition of most likely (maximum likelihood) signal state determined from the sampling value according to a predetermined algorithm, and the reproduction data is generated on the basis of the determined transition of signal state. At this time, maximum likelihood decoded data of front edge and maximum likelihood decoded data of rear edge are independently detected, and the individual maximum likelihood decoded data are synthesized, and data is demodulated.
This PRML reproducing technology is a digital detecting system for operating by using the sampling value, and is completely different from the analog detecting system in the conventional reproduction apparatus mentioned above. Therefore, in such conventional reproduction apparatus, the problem is that the PRML reproducing system which is excellent in detection capability cannot be applied.
Incidentally, in the above detecting system of PRML reproducing technology, a system of reproducing pit-position recorded data by utilizing one side of front edge or rear edge, that is, a pit-position detecting system by applying the PRML has been proposed. In this system, pit-position recorded data can be reproduced, but a sufficient margin is not obtained. The reason is explained below.
FIG. 1 is a graph showing the result of experiment of error rate in the case of reproduction of pit-position recorded data by three-value two-state PR (1, 1) ML. When the sampling phase is near +15%, the error rate is worse than 10.sup.-5. When the error rate allowable range is 10.sup.-5 or less, the phase margin is only about 25%, which is not suited to practical use. The reason of such worsening the error rate is the presence of the transition region of the detection state at this phase position.
FIG. 2 is a graph showing the pit-position detecting state by three-value two-state PR (1, 1) ML. The merge state 1 in the detectable range is y1=+merge, y2=no merge, y3=merge and the merge state 2 is y1=+merge, y2=-merge, y3=no merge and at the turning point from the merge state 1 to 2 (the sampling phase in the vicinity of +15%), the merge states coexist due to noise, and data may not be detected correctly.
Or, if attempted to install the pit-position detecting system by analog configuration in the detecting system of PRML reproducing technology by digital configuration, since only few circuits can be shared, sufficient reduction of size and saving of power are not achieved on the whole.